Typically used, known interferometers have two reflecting mirrors, in some interferometers in the form of retroreflectors. Generally one of the reflecting mirrors is mounted movably in a longitudinal direction, i.e. along the path of the radiation beam. The theory and use of interferometers are thoroughly described in "Fourier transform infrared spectrometry", by Peter R. Griffiths and James A de Haseth, Vol. 83 in "Chemical Analysis". In this specification the term "retroreflector" means an optical device, such as a comer reflector or cat eye, which will reflect an incident light ray in a direction which is parallel to the incident ray, as explained in the above-mentioned reference, e.g. FIG. 4.12 on p.144.
When constructing the arrangement for the movement or longitudinal displacement of the movable mirror, or reflector, great efforts have to be made to ensure controlled displacement. A reliable interferometer shall be substantially insensible to environmental vibrations. A good interferometer will primarily be sensible to vibrations in the same direction as the controlled movement of the movable part.
U.S. Pat. No. 4,383,762 discloses a two-beam interferometer for Fourier spectroscopy designed to be housed in a cryostat aboard a spacecraft, with rigid pendulum structure mounting at least one of the movable retroreflectors in a fully compensated optical system immune to tilt and lateral movement distortions. By this structure the linear displacement is replaced by a pivotal mounting of one or both retroreflectors. Similarly, EP 369 054 describes a pendulum reflector system for a Michelson interferometer. In FIG. 1 of EP 369 054 two retroreflectors are arranged on two perpendicular arms bearing on a common pivot. These interferometers are favourable in that sense that they can be made in such a way that they are only sensible to vibrations in the direction of rotation. Further the weight of the movable parts may be compensated by a balance weight. The driving mechanism must be dimensioned to suppress movements caused by environmental vibrations.
The purpose of the present invention is to provide a simple arrangement for an interferometer said arrangement being a structure which is cheaper to manufacture than the hereto known interferometers, and which simultaneously still has the properties necessary to obtain useful measurements, which are sufficiently accurate for the purpose of the actual measurement.